1. Technical Field
The present invention relates to an automatic gain control apparatus and an automatic gain control method, and more particularly to an automatic gain control apparatus for a wireless receiver having a plurality of receive chains, and an automatic gain control method.
2. Description of the Background Art
Automatic gain control apparatuses are used for data communication over wireless local area networks (WLAN) or the like, in which packet signals are transmitted from one apparatus to another apparatus having one or more receive chains. In such a configuration, the received signal power for each receive chain may fluctuate between a part used for multiple-input multiple-output (MIMO) of the packet signals and a part not used for MIMO of the packet signals. The output level of each receive chain is measured by shifting or chaining preamble signals disposed at the front of the MIMO-part for conducting automatic gain control (AGC) for each one of the receive chains, in which the preamble signals may be time-shifted or frequency-shifted. Thus, for example, JP-4212548-B discloses a configuration to improve output performance of receive chains by minimizing fluctuation in reception power among the receive chains.
JP-4212548-B discloses a configuration in which a plurality of receive chains 203 is connected to a signal processor 202, as shown in FIG. 1. Each one of the receive chains 203 includes an antenna 210, a down converter 211, a variable gain amplifier 212, and an analog-to-digital (AD) converter 213. Further, a gain controller 204 is provided for each of the receive chains 203. Accordingly, the gain is set independently for each one of the variable gain amplifiers 212 of the corresponding receive chain 203.
JP-4212548-B further discloses a configuration in which a plurality of receive chains 303 is connected to a signal processing unit 302 as shown in FIG. 2. Each one of the receive chains 303 includes an antenna 310, a down converter 311, a variable gain amplifier 312, and an AD converter 313. A single gain controller 304 is provided for all the receive chains 303. As a result, the gain of the variable gain amplifier 312 of every one of the receive chains 303 is the same.
However, with the configuration shown in FIG. 1, if the output level of one specific receive chain 203 becomes too small compared to the output level of the other receive chains 203 due to interference or the like, the signal-to-noise ratio (SNR) of the specific receive chain 203 also deteriorates substantially. In such a case, the signal received by the specific receive chain 203 is amplified so that the output level of specific receive chain 203 becomes the same as that of the other receive chains 203.
However, such amplification process also amplifies the noise component in the received signal, and such amplified noise component is input to a later stage unit such as the signal processor 202, thereby degrading signal receiving performance. As such, the receiving performance of the automatic gain control apparatus shown in FIG. 1 deteriorates.
Further, with the configuration shown in FIG. 2, the same gain is set to the variable gain amplifier 312 for every one of the receive chains 303. In such a case, the output levels among the receive chains 303 after gain control may vary substantially due to the effects of interference or the like.
If output levels among the receive chains 303 of the conventional automatic gain control apparatus of FIG. 2 fluctuate, receiving performance deteriorates. For example, an overflow occurs at the receive chain 303 having a high receiving level when the analog-to-digital (AD) conversion is conducted, and a decrease in effective number of bits of signals and resultant increase of quantization noise occurs at the receive chain 303 having a low receiving level.